RANDOM ACCESS COMMUNICATION METHOD BY USE OF CDMA, AND SYSTEM FOR MOBILE STATIONS WHICH USE THE METHOD

Abstract: Each of the mobile stations 12₁ to 12₃ sets delay amounts d₁, d₂ and d₃ at random for every message to be transmitted on the basis of the symbol timing received from the base station in order to give delay to the transmission signal. The base station (11) inversely diffuses the reception signal to extract the standard symbol timing for each of the mobile stations, and allows each demodulator to receive the inverted diffusion output at the extracted timing and to demodulate the reception signal.

Description: TECHNICAL FIELD The present invention relates to a CDMA (Code Division Multiple Access) random access communication method and mobile station equipment which enable a plurality of mobile stations and a base station to communicate using the same frequency and the same spreading code and permit random access to the base station from the mobile stations. The service area of a mobile communication system called a cellular system is split into a number of cells, each of which is provided with a base station. Each cell is assigned at least one control channel, in the case of heavy traffic, two or more control channels, and a plurality of communication channels and each mobile station perform communication through the base station of the cell to which the mobile station currently belongs. Each mobile station under contract to receive services is assigned an identification number, and for example, in the case of sending a call originating signal to the base station over the control channel or sending a speech signal to the base station over the communication channel, the mobile station adds its identification number to the information to be sent. The code division multiple access (CDMA) system that has been employed in the past is a communication system which multiplexes signals to be transmitted at the same frequency, by spectrum spreading them with spreading codes which differ with channels. The system configuration and capabilities of the conventional system are described in detail in a reference literature (R.C. Dixon, "Spread Spectrum Communication System," Jatec Shuppan). Now, a brief description will be given of a direct spreading CDMA system. in Fig. 1 there is shown the configuration of a base station transmitting device in a typical CDMA communication system. N information sequences S1, S2, ..., SN are provided to multipliers 41 1 , 41 2 , ..., 41 N , wherein they are spectrum spread by multiplying each symbol in each information sequence with a corresponding one of spreading codes C1, C2, ..., CN from a spreading code generator 42 which differ from one another. These spread signals are combined by a combiner 43 at the same timing and the combined output is transmitted by a transmitter 44. At the receiving side, though not shown, the received signals are spectrum-despread by the same spreading codes C1, C2, ..., CN as those used at the transmitting side, by which the N information sequences are extracted individually. In this case, pseudo-random noise codes of the same code length (chip number) (hereinafter referred to as PN codes) are used as the respective spreading codes. As is well-known in the art, the PN codes can be generated by, for example, EXCLUSIVE ORing outputs from the last and a desired intermediate stage of a shift register composed of N delay stages, then Inputting the EXCLUSIVE OR into the first stage of the shift register and driving it by a clock of a predetermined period (chip period) Tch. In this instance, a random code (an M-sequence code) of a fixed pattern which repeats with the longest (2 n -1) chip period can be obtained according to the position of the intermediate state to be selected and the initial value to be set in the shift register. 1/Tch is called a chip rate and the period Tc of the M-sequence pattern that is generated is Tc = (2 n -1)Tch. In a digital mobile radio communication system, the transmitting timing of the mobile station is synchronized with a signal from the base station. That is, in one cell, each mobile station demodulates the signal sent from the same base station, then generates basic symbol timing for transmission which is synchronized with the symbol timing for transmission from the base station and transmits thereto a signal in synchronization with the basic symbol timing. The symbol mentioned herein is the minimum unit of information that is transmitted in the radio section. The spreading codes that are needed to construct a large-capacity cellular CDMA system are code sequences which are excellent in auto-correlation characteristic, that is, code sequences which, letting the time offset (the phase difference) between two identical code sequences be represented by τ, have high correlation for only τ = 0 and low correlation for other values of τ. Moreover, the code sequences are those which are excellent in cross-correlation characteristic, i.e. code sequences which have low correlation with other different code sequences (that is, code sequences of cross-correlation values smaller than a fixed value). However, the number of classes of code sequences which satisfy the above-mentioned requirements of spreading codes, such as excellent auto-correlation and cross-correlation, is small and the number of spreading codes in each class is also small. Accordingly, the conventional CDMA communication system in which a different spreading code is assigned to each channel presents a problem that the number of channels available for use is small. Assume that one channel is assigned for random access use, that is, only one spreading code is assigned therefor. As shown in Fig. 2A , a plurality of mobile stations 12 1 through 12 3 transmit to one base station 11 at the same frequency and using the same spreading code in one cell 10 of a mobile communication network. Let the channel that is determined by the frequency and the spreading code be represented by C1. As shown in Fig. 2B , the information to be sent is provided in units of bursts BST and each message MSG is composed of a plurality of bursts BST. A plurality of such messages MSG provide a sequence of signals SS, for example, a sequence of control signals for controlling operations from originating a call over the control channel to the completion of the call originating sequence immediately prior to the transition therefrom to communication over the communication channel. The mobile stations 12 1 and 12 2 send such information randomly, and consequently, different signals of the same frequency and the same spreading code may sometimes be sent at the same timing (in which case the signals overlap each other) (as indicated by clash CLS in Fig. 2B ). In this instance, it is very likely that both signals are not received by the base station receiving device. Moreover, it is customary in the CDMA scheme, with a view to improving the channel quality, to control the transmitting power of each mobile station to provide a constant level in the receiving station. On this account, a capture effect cannot be expected, resulting in increased probability of the signals being not received. Thus, when a plurality of mobile stations use the same spreading code at the same frequency until the end of communication, signals sent from the mobile stations clash with each other and it is very likely that the received signals overlap in the base station, resulting in a substantial reduction of the throughput - this causes serious defects that when the spreading code is assigned to the control channel, the number of mobile stations that can be controlled, that is, the control capacity is small, and that when the spreading code is used for packet signal transmission, the channel capacity is small. If a plurality of control channels are provided in the CDMA scheme through use of a plurality of spreading codes as in the case of using a plurality of control channels in the FDMA scheme, each mobile station selects one control channel which is determined by the identification number of the mobile station. For instance, the identification number is divided by the number of control channels and the control channel corresponding to an integral value of the remainder is selected. In this instance, a particular channel may sometimes be selected with so high frequency that communication over that channel is likely to be interrupted. Conversely speaking, the situation may also arise where a certain channel is selected with a low frequency. Hence, the prior art suffers a problem that the actual signal transmission efficiency is low relative to the transmission capacity provided by all control channels. The document GB-A-2 172 777 discloses a CDMA format used within a network to permit communication between members of a network. Network members are synchronized to each other and are each aware of a predetermined set of spreading codes. A member desiring to transmit data selects a spreading code from the predetermined set. If there are unused spreading codes, one of the unused codes is randomly selected. If all codes are being used, the spreading code of the farthest user is selected. Since all members are synchronized in time to stable clocks, the range of each user is known. All members of the network are always listening and keeping track of detections of other users and spreading codes. The document US-A-4,701,905 discloses a local area network system using a CDMA method. The system includes a repeater for generating a reference synchronizing signal to synchronize the system transceivers connected to the repeater via a sending line and a receiving line, and non-mobile subscriber units, connected to each transceiver. The reference synchronizing signal represents a spreading code that is delayed in each transceiver by a respective delay time uniquely assigned to that transceiver to obtain a delayed spreading code. Because all transceivers are synchronized, if each uses its delayed spreading code, a number of different spreading codes corresponding to the number of transceivers is used at the same time, the individual spreading codes being bit-shifted relative to one another. It is to be noted that in this prior art the spreading codes are delayed, not the signal transmission. The document US-A-4,554,668 discloses a radio communications system comprising at least one master station and a plurality of slave stations in two-way frequency-hopping communication therewith. With the frequency hopping technique information is transmitted in packets in successive bursts at different frequencies. Each slave station has a particular frequency-hopping pattern, with the allowable frequencies being chosen from a predetermined pool of frequencies common to all of the slave stations, and with the master stations being capable of generating the same patterns on the basis of simple data. The bursts are separated by short intervals of dead time in order to facilitate frequency changing, and burst duration is large relative to the dead time and to the propagation time. All the stations on the net change frequency at the same instant. To do this, all the stations must be synchronized to a reference time common to all the elements of the system, with the master stations being directly synchronized to the reference time and with the slave stations synchronizing themselves on data received from the master stations. An object of the present invention is to provide a CDMA communication scheme with which it is possible, in the CDMA mobile communication system, that when a plurality of mobile stations randomly access a base station at the same frequency to transmit thereto signals, the signals are less likely to clash with each other, and hence can be transmitted to the base station with high efficiency. Another object of the invention is to provide mobile station equipment using the CDMA communication scheme. SUMMARY OF THE INVENTION This object is achieved by a method as claimed in claim 1 and a mobile station equipment as claimed in claim 6. Preferred embodiments of the invention are subject-matter of the dependent claims. With the code division multiple access communication scheme according to the present invention, a plurality of mobile stations use the same spreading code and each mobile station transmits a sequence of symbols spectrum-spread by the spreading code while delaying them for a randomly selected period relative to the timing synchronized with received symbols derived from the signal received from the base station, and the base station despreads the received signal from the mobile station by the above-mentioned spreading code to obtain a sequence of symbols and separates them according to the timing of their reception. Thus, according to the present invention, since each mobile station transmits the symbol sequence while delaying it for a randomly selected period relative to the timing of the symbol received from the base station, the probability that the signals sent from the plurality of mobile stations clash with each other becomes low, providing increased transmission efficiency. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram showing a conventional CDMA transmitting device. Fig. 2A is a block diagram showing a base station and mobile stations in one radio zone of a mobile communication network. Fig. 2B is a timing chart showing conventional CDMA random access control. Fig. 3 is a diagram for explaining the method according to the present invention. Fig. 4 is a block diagram illustrating the construction of a mobile station embodying the method of the present invention. Fig. 5 is a block diagram illustrating a specific operative example of a variable delay 35 in Fig. 4 . Fig. 6 is a block diagram illustrating the principal part of the construction of a receiving device of a base station in the present invention. Fig. 7 is a diagram showing an example of the output from a correlation detector 47 in Fig. 6 . BEST MODE FOR CARRYING OUT THE INVENTION A description will be given, with reference to Fig. 3 , of an embodiment according to the first aspect of the present invention. In the conventional CDMA system, at least one control channel and a plurality of communication channels are defined by different spreading codes at the same frequency in each cell of the mobile communication network. In the present embodiment, a base station 11 provided in one cell (a small zone) 10 of the mobile communication network is randomly accessed by a plurality of mobile stations 12 1 , 12 2 and 12 3 in the cell 10, using the same spreading code at the same frequency. In this instance, the mobile stations 12 1 , 12 2 and 12 3 each detect a basic symbol timing of reception from the base station 11 and generate and transmit a signal on the basis of the symbol timing; in the present invention, the symbol transmitting timing differs with the mobile stations. For example, the mobile stations 12 1 , 12 2 and 12 3 use, as their basic symbol timing points for transmission, points delayed for time intervals d 1 , d 2 and d 3 , respectively, behind the basic symbol transmission timing t 0 of the base station 11. The delays d 1 , d 2 and d 3 are selected in the respective mobile stations to be arbitrary values which are each shorter than the period Tc of the spreading code and an integral multiple (including 0-fold) of the chip period Tch. In the present invention, the multipliers 41 1 through 41 N of the transmitting device of the base station shown in Fig. 1 are each supplied with a spreading code for multiplication with each M-symbol information (M being a predetermined integer equal to 1). For example, the mobile stations 12 1 and 12 2 shown in Fig. 3 , for instance, select, as basic symbol transmission timing points, timing t1 and timing t2 which are determined by the delays d 1 and d 2 shorter than one symbol length Ts. As described later on, the delays d 1 , d 2 and d 3 may freely be changed within the range of the spreading code period length Tc for each message composed of a plurality of bursts. For instance, in the case of applying the present invention to the transmission of a control signal over a control channel, the transmission timing is changed for each message, and when the signal clashes with a signal from another mobile station, the provision is made to send the signal again as is well-known in the art. In the case of changing the transmitting timing for each message or for each transmission, the timing is selected so that no signal clash will occur. The mobile stations 12, through 12 3 each have such a construction as shown in Fig. 4 . The electric wave from the base station 11 is received by an antenna 21 and is fed via a duplexer 22 to a receiver 23, by which it is received and amplified to an intermediate-frequency signal, which is provided to, for example, a sliding type correlation detector 24. A code generator 25 generates a spreading code in synchronization with a chip clock CK regenerated by a synchronization controller 26 and provides the spreading code to the correlation detector 24. The correlation detector 24 detects the degree or magnitude of the correlation between the received signal and the spreading code, and the synchronization controller 26 controls the period Tch of the chip clock CK so that the mean output level of the correlation detector 24 increases. As the result of this, the phase of the spreading code that is generated by the code generator 25 is synchronized with the spreading code component contained in the received signal and a symbol sequence, which is a despread signal, is provided at the output of the correlation detector 24. The despread output is demodulated by a demodulator 27 into a DQPSK output, which is provided as the received signal to a terminal 29 via an information controller 28. A transmission signal from a terminal 31 is appended with an identification number in the information controller 28 and fed into a modulator 32, which uses the transmission signal to modulate a carrier into DQPSK form, for example. In this case, the information controller 28 determines the basic symbol timing of the transmission signal in synchronization with basic symbol timing tsy of the received signal from the base station 11 which is obtained in the demodulator 27. The modulated output symbol of the modulator 32 is provided to a spreader 33, wherein it is spectrum-spread by being multiplied, for each chip, by the spreading code of a chip number K and a repetition period Tc which is provided from the code generator 34. The spreading code is generated in synchronization with the chip clock CK from the synchronization controller 26 for each message transmission (upon each occurrence of a setting signal Cs from the information controller 28), and the period length Tc of the spreading code is set to be equal to one symbol length of the modulating signal. The spread output is delayed by a variable delay 35 for an amount preset therein. As referred to previously, the amount of delay takes a value which is smaller than one spreading code period length Tc and is an integral multiple (including 0) of the chip period Tch (= Tc/K); each mobile station selects the delay at will. Assuming that the chip number K of the spreading code is K = 2 n -1, the probability that a certain mobile station selects the same amount of delay as that by another mobile station is 1/(2 n -1). Accordingly, letting the number of mobile stations having started or starting communication at a certain point in time in the same cell 10 be represented by Z, the probability that two of such mobile stations select the same amount of delay, that is, the probability of a signal clash, is (Z-1)/(2 n -1). This probability could be sufficiently reduced by selecting the value n large. This amount of delay is randomly changed for each transmission of one message under the control of the information controller 28. For example, a random generator 36 is provided, and for each message transmission, a random number generated by the random generator 36 is set as the amount of delay in the variable delay 35 by the information controller 28. For example, as shown in Fig. 5 , the variable delay 35 has a shift register 37 composed of m shift stages and its output is provided to an OR circuit 38, whereas the output from the spreader 33 is fed via a gate Go into the OR circuit 38 and provided as well to first, second, ..., m-th shift stages of the shift register 37 via gates G 1 through G m , respectively. The first shift stage is at the side of the OR circuit 35B, and the number m of shift stages is set to a value smaller than the chip number K of the spreading code by one. The random generator 36 randomly generates integers from 0 to m with a fixed period and the random number thus generated is set, by the setting signal Cs from the information controller 28, in a decoder 41 built in the register for each transmission of one message. The decoder 41 enables any one of the gates Go through G m in correspondence with the value set in the decoder. The shift register 37 is shifted in synchronization with the chip clock CK (the chip clock of the period Tch) which is used for the generation of a code by a code generator 34. Accordingly, the output spread signal from the spreader 33 is delayed by the variable delay 35 by a multiple of the generated random number of the chip period Tch. The spread output thus delayed is, as shown in Fig. 4 , provided to a transmitter 37, wherein it is converted to the signal frequency for transmission and power-amplified for transmission, thereafter being transmitted to the base station 11 via the duplexer 22 and the antenna 21. Incidentally, the information controller 28 also effects call originating and call incoming control and sets the communication channel (the spreading code for communication use). In the receiving device of the base station 11, as shown in Fig. 6 , the signals sent from the mobile stations 12 1 through 12 3 are received as a combined wave by an antenna 45 and detected by a quadrature detector 46 to obtain an in-phase component I and a quadrature component Q. These components I and Q are amplified and converted to intermediate-frequency signals, which are applied to a correlation detector 47 formed by a matched filter, for example. In the correlation detector 47, the intermediate-frequency signals are convoluted by a common spreading code generated by a code generator 48, by which the values of correlation with the components I and Q are detected and square sums of the correlated values are provided upon each occurrence of the chip clock CK. For example, as shown in Fig. 7 , the correlation detector 47 provides, at its output, signals SR1, SR2 and SR3 transmitted from the mobile stations 12 1 , 12 2 and 12 3 at timing points t1, t2 and t3 defined by the delays d 1 , d 2 and d 3 , respectively, in the spreading code period Tc. The correlation detected outputs are each applied to a corresponding one of demodulators 491 through 49N and to a basic symbol timing extracting circuit 51. In the timing extracting circuit 51, the basic symbol timing points t1, t2 and t3 of the signals received from the mobile stations 12, through 12 3 are extracted, and at each symbol timing thus extracted, one of the demodulators 491 through 49N operates. That is, the output from the correlation detector becomes identical with the value of correlation between every received signal and the spreading code, and as depicted in Fig. 7 , the output level increases at the points t1, t2 and t3 delayed behind the basic symbol timing for transmission tsy for time intervals d 1 , d 2 and d 3 , respectively, and the signals from the mobile stations are separated in time and demodulated separately by the individual demodulators. Information sequences S1 through SN from the demodulators 491 through 49N are provided to a controller 52, wherein their identification numbers are discriminated and from which the information sequences are sent to an exchange not shown. When an ARD demodulator, described in Japanese Patent Application No. 83947/92 entitled "Spread-Spectrum Receiver," is used as each of the demodulators 491 through 49N, even if delay profiles overlap to some extent, the influence of the overlapping portion can be eliminated - this permits reduction of the delay interval between basic symbol timing points of the respective mobile stations. As described above, according to the present invention, even if the same frequency and the same spreading code are used, the possibility of signals clashing with each other can be reduced very low by using different symbol timing for each mobile station; accordingly, even if the same spreading code is used as a channel for which a number of mobile stations compete, the throughput does not appreciably decrease.

Claims: A communication method which performs communications between a base station (11) and a plurality of mobile stations (12 1 -12 3 ) at the same frequency and using the same spreading code in a code division multiple access communication system, said method comprising the steps: A. each of said mobile stations (12 1 -12 3 ) randomly selects, for each transmission of a message, an amount of delay within the period length of said spreading code, which period length is equal to one symbol length, wherein said delay is an integral multiple of the chip period of said spreading code, and transmits a sequence of symbols spectrum-spread by said spreading code after delaying them by said randomly selected amount on the basis of a timing synchronized with a received symbol obtained from a signal received from said base station; and B. said base station (11) despreads the signal received from said mobile station (12 1 -12 3 ) by said spreading code to obtain a sequence of received symbols and separates said symbols in accordance with the timing of their reception. The method of claim 1, wherein said step A by said each mobile station (12 1 -12 3 ) includes the steps of: 1) generating a basic symbol timing in synchronization with a symbol obtained by receiving a signal sent from said base station (11); 2) generating symbols of the signal to be sent, in synchronization with said basic symbol timing; 3) spectrum-spreading a sequence of symbols of said signal to be sent, by multiplying them by a sequence of chips of said spreading code one after another; 4) delaying said sequence of spectrum-spread symbols by said amount randomly selected by said each mobile station (12 1 -12 3 ) on the basis of said basic symbol timing; and 5) transmitting said sequence of spectrum-spread and delayed symbols. The method of claim 2, wherein said step B by said base station (11) includes the steps of: 1) detecting the degree or magnitude of correlation between signals received as a combined wave from said plurality of mobile stations (12 1 -12 3 ) and said spreading code; 2) extracting the timing of the symbols received from said mobile stations (12 1 -12 3 ) in the period length of said spreading code; 3) selecting and separating said received symbols at said received symbol timing corresponding thereto; and 4) demodulating said separated received symbols to an information sequence. The method of claim 1 or 2, which further includes a step wherein said each mobile station (12 1 -12 3 ) adds the identification number of its own to the information to be sent and modulates it to generate said symbol sequence. The method of claim 4, which further includes a step wherein said base station (11) demodulates said despread received symbol sequence to obtain information sequences and arranges them into information for each mobile station (12 1 -12 3 ) on the basis of identification number contained in each of said information sequences. Mobile station equipment using a method for communication between a base station (11) and a plurality of mobile stations (12 1 -12 3 ) at the same frequency and using the same spreading code in a code division multiple access scheme, said equipment comprising: timing generating means (27) which detects a received symbol from a signal received from said base station (11) and generates a basic symbol timing on the basis of said detected received symbol; information control means (28) which generates the symbol of a signal to be sent, in synchronization with said basic symbol timing; spreading means (33) which spreads a sequence of symbols of said signal to be sent, by said spreading code generated with a fixed repetitive period; delay means (35) which randomly selects, for each transmission of a message, an amount of delay within the period length of said spreading code, which period length is equal to one symbol length wherein said delay is an integral multiple of the chip period of said spreading code, and delays said sequence of spread symbols from said spreading means by said randomly selected amount; and transmitting means (37) which transmits said sequence of spread and delayed symbols. The equipment of claim 6, wherein said delay means (35) comprises: delay select means (36) for randomly selecting one of a plurality of possible delays; and delay setting means (35D, G 0 -G m , 35S) for delaying said sequence of spread symbols by said selected delay. The equipment of claim 7, wherein said spreading code is composed of a predetermined number of chips and said delay means comprises random generating means (36) for generating a random number, a shift register (35S) having a predetermined number of shift stages and driven in synchronization with the generation of each chip of said spreading code, and gate means (G 0 -G m ) connected to the inputs of the respective shift stages of said shift register and the output thereof to provide said sequence of spread symbols from said spreading means to one of said shift stages selected corresponding to said random number or the output of said shift register, whereby said sequence of spread symbols are delayed by an amount corresponding to said random number and outputted from said shift register. The equipment of claim 8, wherein said delay means includes decoder means (35D) which is supplied with said random number and selectively enables the corresponding one of said gate means (G 0 -G m ) to provide therethrough said sequence of spread symbols to the corresponding one of said shift stages or the output of said shift register (35S). The equipment of claim 8 or 9, wherein the number of shift stages of said shift register (35S) is smaller than the number of chips of said spreading code by one and said shift register is driven by a clock of the same period as the chip period of said spreading code.

Inventors: DOUZONO YOUICHI, MATSUMOTO TADASHI, UMEDA NARUMI, DOUZONO, YOUICHI, MATSUMOTO, TADASHI, UMEDA, NARUMI
Applicants: NIPPON TELEGRAPH & TELEPHONE, NTT MOBILE COMMUNICATIONS NETWORK INC.
Application year: 1994
Main classification: H04B7
IPC codes: H04J13, H04B7, H04L12, H04B1